The Cordwood Sauna

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

One of the most important (and too often neglected) components of a sauna is ventilation. If the wood heater is in the stove room with you, it will require air to burn - valuable air, your air. Therefore, you'll want to include a direct vent to the exterior to supply combustion air for the firebox. This can simply be a 6"-diameter stovepipe mortared into the wall like a log end. Place it down at floor level and as close as possible to the air intake for the stove. Other ventilation options should also be included. One trick is to leave an inch of clearance at the bottom of the door. Another is to install a couple of 8" diameter ceramic "thimbles" in the wall, in place of log ends. These thimbles can be purchased at masonry supply yards, where they are sold for use in chimney construction. A log end of about the same diameter as the thimble can be inserted or removed, to allow for air as needed. A little wooden handle eases removal.

Install two such vents on opposite sides of the stove room, one a foot higher than the other. The difference in air temperature in a sauna is about 1 F per inch of height, so the 12° differential helps promote air movement. Air temperature drops a little when the vents are open, but is soon restored when they are closed, thanks to the heat stored in the cordwood walls. It's like being inside a brick oven. Incidentally, temperature stratification in the stove room is why it is good to place the sauna benches at various heights, so that people can choose the temperature they prefer. For example, by occupying the lower bench, children can share a sauna with their parents.

But back to structure. At the Earthwood sauna, we finished the top of the cordwood wall with 16 wooden plates, each 22" long, made from 2 x 8 stock. Eight of these plates act as supports for the radial rafter system, composed of 4 x 8 pine timbers. Every second plate, the ones supporting the rafters, is set 1/2" tower than the previous one, establishing a positive pitch for drainage, west to east.

One of the timbers, he east-west one, goes all the way through the building. Six shorter rafters meet at the center and are fastened to the primary raftter by way of 1/4" steel plates, above and below the intersection, and eight pieces of 1/2" threaded rod tying the metal plates and rafters together in a central hub. The photo and illustration above show how the "snow-blocking" spaces, the gaps between rafters, are filled with 6" thick milled cedar logs and mortar. Regular cordwood masonry would serve just as well at this location.

Our ceiling roof is made of 2 x 6 spruce tongue-and-groove silo staves, but any 2 x 6 planking (or even 3/4" plywood) would do. The only caveat is to avoid very pitchy material, as the pitch will bleed out of the wood at the 200°F or higher temperatures sometimes reached at ceiling level.

The Earth Roof

Although an earth roof is not absolutely necessary, it is aesthetically pleasing, long-lasting and is a sauna tradition in Finland. Our roof layers, above the planking, are as follows: 1) The waterproofing membrane. We use the Bituthene® 4000 waterproofing membrane made by W.R. Grace. Installation is easy: Just pull the backing paper off the 36"-wide rolls and press it onto the wooden deck. Detailed instructions appear on the packaging. 2) An inch of Dow Styrofoam® or equivalent extruded polystyrene insulation. 3) A layer of 6-millimeter black polyethylene, the base of the all-important drainage layer. 4) Two inches of #2 crushed stone, the drainage layer. 5) A layer of loose hay or straw, which keeps the drainage layer clear of soils from above. 6) Eight inches of earth planted to grass or sods cut from a field. A roof constructed in this way should last 100 years or forever, whichever comes first.

RSS | Comments | E-mail | Print